LEDs are energy-efficient and environmentally friendly and feature high lighting efficiency and long working life. As such, LEDs have seen more extensive applications lighting installations in general and specifically in road illumination as a new generation of green, energy-efficient light sources. LED street lamps have become a leading choice in the transformation of road lighting for energy conservation. However, from the perspective of illumination. LED street lamps still face technical problems in four areas, namely lighting efficiency, light distribution, light attenuation and color temperature. Considerable improvements have been made in the lighting efficiency, light distribution, and light attenuation of LED street lamps due to rapid developments in LED semiconductor techniques, secondary light distribution technology, and heat-radiating technology.
For example, the various secondary optical lens types such as those having a free-curved-face peanut shape, a saddle shape or an asymmetric curved face for polarization can distribute light emitted by LED into highly-efficient uniform light patches of a rectangular shape. The curved surface for light distribution adopts a bat-wing shape is well adapted to satisfying the design standards of urban road illumination in China.
However, until now there has been no satisfactory solution to the color temperature differences (i.e., color differences) of LED street lamps. The uneven application of fluorescent powders on the light-emitting surface of LED chips and the color differences inherent to the secondary optical lens will normally generate different color temperatures in the middle and at the edges of the projected light patches. The light patches are bluish with a higher color temperature in the middle, but yellowish with a lower color temperature at the edges. In addition, color temperature is an important parameter affecting the performance of LED street lamps, and its spatial distribution is highly significant for product performance.
The relevant color temperature refers to the temperature of a black-body radiator that is most similar to the color of the same brightness stimulus. The relevant color temperature difference distinguishable by the human eye may be as low as 50-100K, compared to up to several hundred K in the differences in the spatial distribution of relevant color temperatures of LED street lamps. The lens with color differences will generate highly distinctive yellow-and-white “optical zebra crossings” on the road surface, and hence severely affect the visual effect of the street lamp.